1. Field of the Invention
The present invention relates to radar detection circuits and more particularly to baseband carrier detection circuits for expanded time ranging systems. The invention can be used to accurately detect the time of occurrence of pulsed RF echoes for sampling radar, TDR (Time Domain Reflectometry) and laser sensors.
2. Description of Related Art
Short range, high resolution pulse-echo ranging systems, such as wideband and ultra-wideband pulsed radar and pulsed laser rangefinders often transmit a short sinusoidal burst on the order of 1-nanosecond in duration and consisting of about six cycles of RF. Radars having these parameters can be found in, for example, commercial pulse-echo rangefinders used to determine liquid levels in tanks. These radars operate in an expanded time mode, whereby the transmit pulse rate is slightly higher than the receiver gate, or sampling, rate, to produce a stroboscopic slow motion sampling effect, i.e., a down-sampling, time expansion effect.
The stroboscopic effect produces detected output pulses that resemble the received RF echo pulses, but occur on a vastly expanded time scale. Time expansion factors of 100,000 to 1-million are common. Accordingly, RF echo pulses having a 6 GHz carrier frequency produce sampled output echo pulses having a 6 kHz carrier frequency. These 6 kHz pulses are expanded time replicas of the RF echo pulses. At 6 kHz, pulse detection or other processing is vastly easier. Examples of expanded time radar architectures are disclosed in U.S. Pat. No. 6,191,724, “Short Pulse Microwave Transceiver,” by the present inventor, Thomas E. McEwan, and in U.S. Pat. No. 6,414,627, “Homodyne Swept Range Radar,” also by the present inventor.
A problem arises in precisely detecting pulsed RF produced by these systems. One sinewave cycle looks very much like the next within a sinewave burst, so a detector has difficulty detecting a particular sinewave cycle within the burst. For best ranging accuracy, the detector must consistently detect one specific cycle within the echo burst. Preferably, one particular point on a selected sinewave cycle must be detected.
One approach to the detection problem is a fixed threshold detector that triggers on the first sinewave cycle to cross the threshold. Unfortunately, variations in received signal amplitude make this approach unattractive since cycle jumps are inevitable as amplitude varies with target range, aspect angle and size.
Another approach is to detect the envelope of the sinusoidal burst and then threshold detect the envelope, with the detection time occurring at a threshold crossing. Alternatively, the envelope's time-of-peak (TOP) can be detected. Yet another technique is constant fractional maximum detection (CFD), wherein a peak detector detects peak amplitude, which is coupled through a voltage divider to set a tracking detection threshold at a constant fraction of the pulse maximum. In all these cases, the envelope is slower and of lower bandwidth than an individual cycle within the burst, and so detection accuracy suffers accordingly. A ten fold reduction in accuracy is not uncommon with these envelope detection techniques.
An automatic sinusoidal burst detection technique is disclosed in U.S. Pat. No. 6,137,438 “Precision Short Range Pulse-Echo Systems with Automatic Pulse Detectors,” by the present inventor. A peak detector detects peak envelope amplitude and sets a fraction of this peak—as a form of a CFD—as the threshold for the next repetition of the expanded time sinusoidal burst. Thus, a consistent detection point can be set on a selected cycle in the burst. This approach, while effective, has two limitations. First, rapid pulse-to-pulse variations are not tracked since the peak of one pulse is used to set a threshold on the next pulse. Second, detection does not occur at the zero axis crossings of the sinewaves where the voltage rate of change is fastest and detection can be accomplished with a minimum of noise and error. Thus, a better approach is needed for varying targets and for higher accuracy.
U.S. Pat. No. 5,457,990, “Method and Apparatus for Determining a Fluid Level in the Vicinity of a Transmission Line,” by Oswald et al, discloses a detection technique employing a threshold detector to define an analysis window of time. Whenever pulse amplitude exceeds the threshold, a TOP detector is enabled and detection occurs. The analysis window gates out noise outside the window. However, the '990 patent fails to teach detection of multiple sinusoids in a burst—it is limited to single transients. Multiple cycles within a burst present an ambiguity as to which cycle to detect, and this problem is not addressed in the '990 patent. Furthermore, the '990 patent is limited to an analysis window derived from a single transient above threshold. A sinusoidal burst is not a transient. Thus, an entirely new technique is needed.